31.3 ). The rectum lacks circular muscle and is surrounded only by longitudinal muscle fibers. It is a reservoir wherein feces can be stored before defecation. Muscular contractions also form functional “valves” in the rectum that retard movement of feces and are important in delaying the loss of feces until it is convenient, at least in adults. The rectum in turn joins the anal canal, distinguished by the fact that it is surrounded not only by smooth muscle but also by striated (skeletal) muscle. The combination of these muscle layers functionally accounts for two key sphincters that control evacuation of solid waste and flatus from the body. The internal anal sphincter is composed of a thickened band of circular muscle, whereas the external anal sphincter is made up of three different striated muscle structures in the pelvic cavity that wrap around the anal canal. These latter muscles are distinctive because they maintain a significant level of basal tone and can be contracted further either voluntarily or reflexively when abdominal pressure increases abruptly (e.g., when lifting a heavy object).
Hirschsprung’s disease isaconditioninwhichasegmentofthecolonremainspermanentlycontractedandresultsinobstruction.Itistypicallydiagnosedininfancyandaffectsupto1in5000livebirthsintheUnitedStates.Thebasisofthediseaseisfailureoftheentericnervoussystemtodevelopnormallyduringfetallife.Duringorganogenesis,cellsdestinedtobecomeentericneuronsmigrateoutfromtheneuralcrestandpopulatethegutsequentiallyfrommouthtoanus.Insomeindividualsthismigrationterminatesprematurelybecauseofabnormalitiesinthemechanismsthatwouldotherwisedrivethisprocess.Mutationsinglial-derivedneurotrophicfactorandendothelinIII,aswellasintheirreceptors,havebeendescribedinindividualssufferingfromthisdisease,andtheaffectedsegmentcompletelylackstheplexusesoftheentericnervoussystemandassociatedganglia.ArelativedeficiencyofinterstitialcellsofCajalisalsoseenintheaffectedsegment,andoverallcontrolofmotilityismarkedlyimpaired.Inmostindividualsthesymptomscanbecompletelyalleviatedbysurgicalexcisionoftheaffectedsegment.
Contraction of the smooth muscle layers in the proximal part of the colon is stimulated by vagal input as well as by the enteric nervous system. On the other hand, the remainder of the colon is innervated by the pelvic nerves, which also control the caliber of the internal anal sphincter. Voluntary input from the spinal cord via branches of the pudendal nerves regulates contraction of the external anal sphincter and muscles of the pelvic floor. The ability to control these structures is learned during toilet training. This voluntary control distinguishes the anal canal from most of the GI system, with the exception of the striated muscle in the esophagus that regulates swallowing.
Consistent with its primary function, the two predominant motility patterns of the large intestine are directed not to propulsion of the colonic contents but rather to mixing of the contents and retarding their movement, thereby providing them with ample time in contact with the epithelium. Two distinctive forms of colonic motility have been identified. The first is referred to as short-duration contractions, which are designed to provide for mixing. These contractions originate in the circular muscle and are stationary pressure waves that persist for 8 seconds on average. Long-duration contractions, in contrast, are produced by the taeniae coli, last for 20 to 60 seconds, and may propagate over short distances. Notably, however, propagation may move orally as well as aborally, particularly in the more proximal segments of the colon. Both motility patterns are thought to originate largely in response to local conditions such as distention. Note that the basal electrical rhythm that governs the rate and origination sites of smooth muscle contraction in the small intestine does not traverse the ileocecal valve to continue into the colon.